What is the Last Trump?
1Corinthians 15:52 in a moment, in the twinkling of an eye, at the last trump: for the trumpet shall sound, and the dead shall be raised incorruptible, and we shall be changed.
1Thessalonians 4:16 For the Lord himself shall descend from heaven, with a shout, with the voice of the archangel, and with the trump of God: and the dead in Christ shall rise first;
1st thing we learn is that it is "the trump of God". The Old Testament is full of references for God's people to blow the trumpet, they do that on new moons, on sabbaths, on feast days and yes on "the feast of trumpets" but this trumpet is not any of those. This is not man blowing the trumpet, it is "the trump of God". The trumpet was used to call for an assembly, to warn of an enemy coming, to announce the coming king, to celebrate the wedding of the king, to celebrate the coronation of the king, in short it is what the Lord told us -- we are His witnesses. Our testimony is a trumpet blast. Not only so but the shofar is a rams horn, it reminds God of mercy. When Abraham was told to sacrifice his son God provided a ram instead. That was mercy. The first trump refers to one of those rams horns, the last trump to the other.
So then, what was the first Trump?
Exodus 19:16 And it came to pass on the third day, when it was morning, that there were thunders and lightnings, and a thick cloud upon the mount, and the voice of a trumpet exceeding loud; and all the people that were in the camp trembled. 17 And Moses brought forth the people out of the camp to meet God; and they stood at the nether part of the mount. 18 And mount Sinai, the whole of it, smoked, because Jehovah descended upon it in fire; and the smoke thereof ascended as the smoke of a furnace, and the whole mount quaked greatly. 19 And when the voice of the trumpet waxed louder and louder, Moses spake, and God answered him by a voice.
When God gave the law to the children of Israel it began with the first Trump of God. You can be sure that the sounding of the last trump will look like this. Thunders, lightnings, the mountain will smoke, etc. In this situation people were scared, no one is going to be able to blow a trumpet while this is going on, yet the Feast of Trumpets is when you blow the trumpet. We are the choir, God is the soloist. While He blows the trumpet we are silent. The same will be true of the last trump. Think of the Feast of Trumpets as the choir coming in on the chorus, God does the verse, then we all respond with the chorus. God raptures the church, judges the Great Babylon while we all shake and are silent, then when Feast of Trumpets comes we have regained our composure and can all join in on the chorus -- the king is coming, this is the day of the Lord, repent, seek His face, call on Him.
Is the 100th trump the last trump?
On the feast of trumpet they blow the trumpet 100 times, the 100th time is called the last trumpet. Is that the "Last Trump"? No. Only once before have we seen "the trump of God" it was when He gave the law. God blows the trumpet twice, once when He gave the law and once again at the end of the age and start of the Day of the Lord. Think of these 100 trumpet blasts as pieces to a puzzle, we are putting this puzzle together. The last trumpet on that day is when we have put the puzzle together. It is the glory of God to conceal a matter and the glory of kings to search a matter out.
Does the Feast of Trumpets depict the rapture?
No. The blowing of the trumpets is an alarm (depicts the start of the tribulation), it announces the coming of the king (that is the day of the Lord), it signifies that the doors of heaven have been opened (that refers to judgement that will be poured out on the Earth). The trumpet blasts are from people to remind God as He prepares to judge us to remember mercy (that is appropriate for those left behind after the rapture).
It announces the wedding of the Lord and just like the parable of the ten virgins this does refer to the rapture, but wait, there is a cry in the midnight hour that the Bridegroom cometh, they light their lamps and go out to meet Him. That is the rapture, that is not the wedding! That midnight cry is the last trump, it wakes the dead, they go out to meet Him, but then some realize they don't have oil and they have to go buy the oil and when they return the door to the wedding is closed. Look at the timeline -- midnight cry (last trump) --> dead are raised --> some are foolish and are told to go buy oil from those who sell (the two witnesses during the tribulation) --> when they return the door is already closed. From this we learn that the midnight cry precedes the wedding, the tribulation begins, the wedding begins, and those in the tribulation are locked out. So yes, the last Trump of God and the Feast of trumpets are very close together but they are not the same.
Finally, the Feast of Trumpets is a Memorial
Leviticus 23:24 Speak unto the children of Israel, saying, In the seventh month, on the first day of the month, shall be a solemn rest unto you, a memorial of blowing of trumpets, a holy convocation.
Just like the rebuke from the Lord to the church in Sardis.
Revelation 3: 3 Remember therefore how thou hast received and didst hear; and keep it, and repent.
This is a word to those who are left behind.